1. Field of the Invention
The present invention relates to a display device in which a thin film transistor (hereinafter, abbreviated as “TFT”) is used as a switching element.
2. Description of the Related Art
Recently, a TFT in which a polycrystalline silicon film is used as an active layer is being developed as a driver element or a pixel driver element for various kinds of display devices such as an active-matrix liquid crystal display device (hereinafter, abbreviated as “LCD”).
Hereinafter, a reflection type LCD having conventional TFTs will be described. A reflection type liquid crystal display device is a display device in which light entering from the side of the viewer is reflected by a reflective display electrode so as to enable a display to be viewed.
FIG. 8 is a plan view of a TFT of a display pixel portion of the conventional art, and FIG. 9 is a section view of an LCD using a TFT and taken along the line E—E in FIG. 8.
As shown in FIG. 8, a TFT of a display pixel portion is disposed in the vicinity of an intersection of a gate signal line 51 through which a gate signal is supplied, and a drain signal line 52 through which a video signal is supplied. The source 13s of the TFT is connected to a reflective display electrode 19. The reflective display electrode 19 is not formed on the TFT.
Referring to FIG. 9, the structures of the TFT and the LCD will be described.
Gate electrodes 11 made of a refractory metal such as chromium (Cr) or molybdenum (Mo), a gate insulating film 12, and an active layer 13 configured by a poly-silicon film are sequentially formed on an insulative substrate 10 which is made of quartz glass, non-alkali glass, or the like.
In the active layer 13, disposed are channels 13c above the gate electrodes 11, and a source 13s and a drain 13d which are respectively formed on both sides of the channels 13c by ion implantation using stopper insulating films 14 on the channels 13c as a mask.
A interlayer insulating film 15 in which an SiO2 film, an SiN film, and an SiO2 film are laminated is formed over the whole face of the gate insulating film 12, the active layer 13, and the stopper insulating film 14. A drain electrode 16 is formed by filling a metal such as Al into a contact hole which is opened correspondingly with the drain 13d. A planarizing insulating film 17 which is made of an organic resin or the like and which planarizes the surface is formed over the whole face. In the planarizing insulating film 17 and the interlayer insulating film 15, a contact hole is opened at a position corresponding to the source 13s. A reflective display electrode 19 which is made of a reflective material such as Al and contacted with the source 13s via the contact hole is formed on the planarizing insulating film 17. An alignment film 20 which is made of an organic resin such as polyimide and which is used for aligning a liquid crystal 21 is formed on the reflective display electrode 19. At this time, the reflective display electrode 19 is not formed on the TFT.
The insulative substrate 10 having TFTs which have been produced as described above, and a counter electrode substrate 30 having a counter electrode 31 and an alignment film 32 which are opposed to the substrate 10 are bonded in periphery together by a sealing adhesive agent 23. The liquid crystal 21 is filled into a gap formed between the substrates. Polarizing plates 33 are respectively applied to the outsides of the substrates 10 and 30, thereby completing the LCD.
FIG. 10 shows characteristics of the TFT. The abscissa indicates the gate voltage Vgs, and the ordinate indicates the drain current Ids.
Since the reflective display electrode 19 does not exist above the channels of the TFT, the TFT exhibits characteristics in which, as indicated by the solid line in FIG. 10, the off-state current does not flow when the gate voltage Vgs is 0 V. By contrast, in the case where the reflective display electrode is formed so as to cover also the whole face of the TFT, the characteristics are changed as indicated by the broken line in FIG. 10.
This is caused by the following reason. A voltage is applied to the reflective display electrode 19 disposed above the channels 13c. Charges are generated by an electric field due to the voltage, and so-called back channels are generated with respect to the channels 13c. 
When such a TFT is used in an LCD, the reflective display electrode may be elongated to extend above the TFT in order to improve the aperture ratio. This causes the threshold voltage to be changed in a decreasing direction, and hence the off-state current is increased as shown in FIG. 10. As a result, there arise drawbacks that a bright defect in which a pixel is always bright, or an unlit defect in which a pixel is always unlit occurs and an excellent display cannot be obtained, and that the threshold voltages is varied among TFTs and a display which is uniform in brightness over the screen cannot be obtained.
The invention has been conducted in view of the drawbacks of the conventional art. It is an object of the invention to provide a display device in which a, TFT is shielded against an electric field caused by a pixel electrode above the TFT to stabilize the threshold voltage of the TFT, thereby enabling a display in which defects such as a bright defect are reduced, brightness is uniform over the screen, and the aperture ratio is improved.